The present invention relates to mobile radio geolocation systems. More particularly, the present invention relates to CDMA geolocation systems capable of determining the location of a mobile CDMA transceiver operating in a CDMA communication system.
Mobile radio communication systems are well known in the art. Such systems typically use a plurality of base stations for broadcasting signals to, and maintaining voice or data communications with, mobile radio equipment. Most such mobile radio equipment, which usually are actually transceivers in that they are typically enabled to both broadcast and receive, are individually identified by a code or call number. Once turned on, each radio transmits an identifying signal that includes the code or call number so that the radio may be contacted if a call is directed to it. That is, the user of the radio does not have to be actively engaged in communication on the radio for an identifying signal of that radio to be received by the base stations.
Over time, it has become increasingly apparent that the ability to locate the source of a mobile radio would be advantageous for a variety of reasons. Of paramount importance is the ability to locate an injured person or law enforcement officer in distress. Other advantageous reasons relate to locating the origin or source of an illegally operated mobile radio.
In most mobile radio communication systems there is a high likelihood that the mobile radio signal can be received at multiple dispersed sites, such as dispersed base stations, allowing triangulation through lines of bearing or time difference of arrival (TDOA) hyperbolas. However, certain characteristics of code-division multiple access (CDMA) communication systems present special problems when implementing a geolocation system for determining the location of a mobile CDMA transceiver.
One main difference between CDMA communication systems and other mobile radio communication systems, such as FDMA and TDMA, is the received signal strength of the mobile radio emission at multiple base stations, which is critical for a triangulation-based geolocation system. Conventional communication systems typically use higher transmission power from the mobile radios, thus increasing the chances that the emission will be received and recoverable at multiple base stations. In the CDMA standard, mobile transceiver transmission power is kept at the minimum level required to ensure reliable communication with one base station. This is done in order to maximize the capacity of the CDMA communication system.
As a result of power control in a CDMA communication system, mobile CDMA transceivers are often transmitting at very low power levels. This is especially apparent as they move closer to a base station. Due to what is termed the xe2x80x9cnear-farxe2x80x9d affect, mobile CDMA transceivers close to a base station need only transmit at low power. Those that are far away from a base station need to transmit at higher levels such that all emissions are optimally at equal power when they arrive at the base station.
In terms of the geolocation of CDMA signals, this power control severely limits the applicability of conventional geolocation methods, since mobile CDMA transceiver emissions are often only easily recoverable at one base station. Thus, new techniques for receiving mobile CDMA transceiver emissions at multiple base stations are needed in order to use conventional triangulation techniques for systems utilizing emerging CDMA air standards such as IS-95.
Unlike the air standards applicable to FDMA and TDMA systems, IS-95 allows multiple users to share a common frequency spectrum simultaneously, by assigning each system user a unique spread spectrum spreading code. A spread spectrum system makes use of a sequential noise-like signal structure, for example P.N. (pseudo-noise) codes, to spread the normally narrow band information signal over a relatively wide band of frequencies. The receiver correlates these signals to retrieve the original information signal.
A variety of triangulation-based CDMA geolocation systems have been proposed in U.S. Pat. No. 5,508,708, issued to Ghosh et al, U.S. Pat. No. 5,736,964, issued to Ghosh et al., U.S. Pat. No. 5,675,344, issued to Tong et al., U.S. Pat. No. 5,365,544, issued to Schilling, U.S. Pat. No. 5,506,864, issued to Schilling, and U.S. Pat. No. 5,228,056, issued to Schilling. However, these systems do not address how to overcome the low power characteristic of the received mobile CDMA transceiver emissions without requiring system intervention, such as transmission of special sequences or use of power up functions by the mobile CDMA transceivers. In addition, these systems to not account for frequency offsets between the transmitting and receiving hardware due to tuning error and doppler frequency shifts imposed by transmitter motion. Even slight differences in frequency will render these systems largely ineffective.
Accordingly, it would be advantageous to provide means for geolocating the position of a spread spectrum coded radio frequency emission, such as a CDMA radio signal, which addresses problems resulting from the low power characteristics of received mobile CDMA transceiver emissions without requiring special intervention by the communication system.
In accordance with the present invention, a CDMA geolocation system capable of determining the location of a mobile CDMA transceiver operating in a CDMA communication system includes at least one mobile transceiver capable of spread spectrum coded radio frequency emissions and communication with a plurality of base stations. Each base station is capable of receiving the spread spectrum coded radio frequency emissions from the mobile transceiver.
The system also includes means for synchronizing the base stations to the mobile transceiver in time and means for determining the geolocation of the mobile transceiver based on times of arrival of the spread spectrum coded radio frequency emission received at the base stations.
Each of the base stations preferably comprises means for identifying information bits of interest from orthogonal sequences of information bits, such as Walsh codes, in the spread spectrum coded radio frequency emission received at the base station and means for determining a time of arrival of the spread spectrum coded radio frequency emission received at base station.
The means for determining the time of arrival of the spread spectrum coded radio frequency emission received at the base station comprises means for dividing the spread spectrum coded radio frequency emission received at the base station into a plurality of stages, despreader/demodulator means for despreading/demodulating each of the stages into a plurality of Walsh codes, and calculating means for calculating the fast Fourier transform of each of the stages to produce a time-frequency cross ambiguity function. The purpose of which is to compensate for frequency offsets imposed by motion induced doppler frequency shifts, or tuning frequency mismatches between the transmitter and receiver hardware. A means for interpolating a selected peak in the time-frequency cross ambiguity function for determining the time of arrival is desirably also included.
The geolocation system further includes a geolocation control unit for determining a primary base station from the plurality of base stations. The primary base station is the base station in active communication with the mobile transceiver.
The geolocation control system includes means for routing the identified Walsh codes of interest from the primary base station to the secondary base stations, which include all of the base stations except the primary base station. Upon receiving the routed identified Walsh codes of interest, each of the secondary base stations identifies the Walsh codes of interest in the spread spectrum coded radio frequency emission it receives based on the routed identified Walsh codes of interest. The geolocation control system also includes means for instructing each of the base stations to store spread spectrum coded radio frequency emissions received at the base station.
The means for determining time of arrival in each of the secondary base stations comprises despreader/demodulator means for despreading/demodulating each of the stages into Walsh codes at over sampled chip offsets. The means for dividing divides the spread spectrum coded radio frequency emission into M stages wherein:   M  =                    (                  2          ⁢                      d            /            c                          )            Tc        *    N  
Tc=Chip Duration (seconds)
c=speed of light (3*108 m/s)
d=maximum expected propagation distance (meters)
N=chip over sampling rate
The means for identifying a code of interest from the emission received at each base station comprises means for extracting Walsh symbols from the radio frequency emission and means for identifying the Walsh symbol having the largest amplitude.
In one form of the invention, each of the base stations is divided into a plurality of sectors, each sector being capable of receiving spread spectrum coded radio frequency emissions from the mobile transceiver and the geolocation system includes means for determining which sector of each base station receives the spread spectrum coded radio frequency emission. In a form of the invention where the spread spectrum coded radio frequency emission is received by only two base stations, each of the base stations having position coordinates and the first base station is designated the origin, the estimated position of the mobile transceiver is determined as follows:       y    ~    =                    4        ⁢        b        ⁢                  xe2x80x83                ⁢        γ            ±                                    16            ⁢                          b              2                        ⁢                          γ              2                                +                      16            ⁢                          (                                                a                  2                                +                                  b                  2                                            )                        ⁢                          (                                                4                  ⁢                                      a                    2                                    ⁢                                      r                    1                    2                                                  -                                  γ                  2                                            )                                                          -        8            ⁢              (                              a            2                    +                      b            2                          )            xe2x80x83{overscore (x)}=xc2x1{square root over (r12xe2x88x92{overscore (y)})}1
xcex3=(r22xe2x88x92r12xe2x88x92a2xe2x88x92b2)
(a,b)=x and y coordinates of second base station (first base station is at the origin)
({overscore (x)},{overscore (y)})=estimated position of mobile transceiver
r1=range from first base station
r2=range from second base station
The present invention also includes a method of ascertaining the geolocation of a mobile transceiver capable of spread spectrum coded radio frequency emission and which is in communication with a plurality of base stations. The method comprises the steps of receiving spread spectrum coded radio frequency emissions from the mobile transceiver at the plurality of base stations, synchronizing the base stations to the mobile transceiver in time, extracting Walsh codes from a spread spectrum coded radio frequency emissions received at each base stations, determining a primary base station based on the extracted Walsh codes and identifying from the extracted Walsh codes Walsh codes of interest in the spread spectrum coded radio frequency emissions received at the primary base station, forwarding the Walsh codes of interest to the secondary base stations, determining the times of arrival of the spread spectrum coded radio frequency emission received at each of the base stations, and determining the geolocation of the mobile transceiver utilizing the determined times of arrival. The primary base station is the base station in active communication with the mobile transceiver and the secondary base stations comprise all of the base stations except the primary base station.
The step of determining the times of arrival of the spread spectrum coded radio frequency emission received at the secondary base stations further comprises the steps of dividing the received spread spectrum coded radio frequency emission into a plurality of stages, despreading/demodulating each of the stages, into the extracted Walsh codes, calculating the fast Fourier transform of each of the stages to produce a time-frequency cross ambiguity function, and determining the times of arrival of the spread spectrum coded radio frequency emission received at each of the secondary base stations based on said time-frequency cross ambiguity function.
The step of determining the times of arrival of the spread spectrum coded radio frequency emission received at each of the secondary base stations further comprises the step of interpolating a selected peak in the time-frequency cross ambiguity function and determining the times of arrival based on the selected peak.
The step of determining the times of arrival of the identified spread spectrum coded radio frequency emission of interest preferably also includes the step of despreading/demodulating each of the stages into the extracted Walsh codes at over sampled chip offsets for each of the secondary base stations. The step of identifying the Walsh codes of interest further comprises identifying extracted Walsh codes having the largest amplitude.
The step of determining the geolocation of the mobile transceiver may further comprise the steps of determining the range differences from the differences in times of arrival measured at the plurality of base stations, and calculating the geolocation of the mobile transceiver based on these differences. Alternatively, calculating the geolocation of the mobile transceiver may be accomplished using the ranges between the mobile transceiver and the plurality of base stations utilizing the determined times of arrival, and calculating the geolocation of the mobile transceiver based on the ranges between the mobile transceiver and the plurality of base stations.
Further objects, features and advantages of the present invention will become apparent from the following description and drawings.